En Route to a Unified Model for Photoelectrochemical Reactor Optimization. II–Geometric Optimization of Perforated Photoelectrodes

Franky E Bedoya-Lora,Geoff H Kelsall,Anna Hankin

doi:10.3389/fceng.2021.749058

Franky E Bedoya-Lora, Geoff H Kelsall + Show 1 more

Open Access

https://doi.org/10.3389/fceng.2021.749058

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Abstract

Results have been reported previously of a model describing the performance of photoelectrochemical reactors, which utilize semiconductor | liquid junctions. This model was developed and verified using SnIV-doped α-Fe2O3 as photoanodes. Hematite films were fully characterized to obtain parameter inputs to a model predicting photocurrent densities. Thus, measured photocurrents were described and validated by the model in terms of measurable quantities. The complete reactor model, developed in COMSOL Multiphysics, accounted for gas evolution and desorption in the system. Hydrogen fluxes, charge yields and gas collection efficiencies in a photoelectrochemical reactor were estimated, revealing a critical need for geometric optimization to minimize H2-O2 product recombination as well as undesirable spatial distributions of current densities and “overpotentials” across the electrodes. Herein, the model was implemented in a 3D geometry and validated using solid and perforated 0.1 × 0.1 m2 planar photoanodes in an up-scaled photoelectrochemical reactor of 2 dm3. The same model was then applied to a set of simulated electrode geometries and electrode configurations to identify the electrode design that would maximize current densities and H2 fluxes. The electrode geometry was modified by introducing circular perforations of different sizes, relative separations and arrangements into an otherwise solid planar sheet for the purpose of providing ionic shortcuts. We report the simulated effects of electrode thickness and the presence or absence of a membrane to separate oxygen and hydrogen gases. In a reactor incorporating a membrane and a photoanode at 1.51 V vs RHE and pH 13.6, an optimized hydrogen flux was predicted for a perforation geometry with a separation-to-diameter ratio of 4.5 ± 0.5; the optimal perforation diameter was 50 µm. For reactors without a membrane, this ratio was 6.5 and 8.5 for a photoanode in a “wired” (monopolar) and “wireless” (photo-bipolar) design, respectively. The results and methodologies presented here will serve as a framework to optimize composite photoelectrodes (semiconductor | membrane | electrolyte), and photoelectrochemical reactors in general, for the production of hydrogen (and oxygen) from water using solar energy.

Highlights

Far, the bulk of the research on photoelectrochemical systems for splitting water with solar energy has focused on material developments (Chen et al, 2010; Tachibana et al, 2012; Moss et al, 2021), aiming to conceive materials that would be: 1) efficient in converting photons to chemical product(s), 2) economical to fabricate and 3) chemically and mechanically durable
Integrated photoelectrochemical devices (IPECs), in which the electrolysis is powered by a solar cell/PV embedded in the device, but in which the PV is either protected from the electrolyte by an interposed layer to prevent photoelectrode corrosion (Reece et al, 2011; Turan et al, 2016) or is integrated with the electrolyser by some other means, such as thermally (Tembhurne et al, 2019)
In this study we focus on the influence of photoelectrode geometry on photoelectrochemical reactor performance

Summary

Introduction

The bulk of the research on photoelectrochemical systems for splitting water with solar energy has focused on material developments (Chen et al, 2010; Tachibana et al, 2012; Moss et al, 2021), aiming to conceive materials that would be: 1) efficient in converting photons to chemical product(s), 2) economical to fabricate and 3) chemically and mechanically durable These materials are typically synthesized and tested at small scale, with electro-active areas ≤1 cm (Khaselev and Turner, 1998; Rocheleau et al, 1998; Kelly and Gibson, 2006; Jia et al, 2016; Bedoya-Lora et al, 2021). Photoelectrochemical devices (PECs), with semiconductor | liquid junction(s) in which the semiconductor(s) often function(s) as catalyst (Brillet et al, 2012; Liu et al, 2013; Shaner et al, 2013; Vilanova et al, 2018)

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